Asphalt may be characterized as an organic cementitious material in which the predominant constituents are bitumens as they may occur in nature or as they may be produced as byproducts in petroleum refining operations. Asphalt materials and the standards to be applied in asphalt paving applications are described in the booklet entitled SUPERPAVE Series No. 1 (SP-1) “Performance Graded Asphalt Binder Specification and Testing,” 3rd Edition, 2003, published by the Asphalt Institute, Research Park Drive, P.O. Box 14052, Lexington, Ky. 40512-4052. Asphalts can generally be characterized as a dark brown or black solid or highly viscous liquid which incorporates a mixture of paraffinic and aromatic compounds and various heterocyclic compounds containing Group 15 or 16 elements (new notation), such as nitrogen, oxygen or sulfur. Typical analyses of asphalt cements employed in forming asphalt concretes are disclosed in the aforementioned SUPERPAVE booklet in Chapter 1 under the subheading “Chemical Composition of Asphalt” found on pages 3-6.
Asphalt paving materials based upon asphalt binder or “cements” and aggregate mixtures, commonly referred to as “asphalt concrete” or macadam, are used in many applications such as in the resurfacing of streets, parking lots and the like which are subject to vehicular traffic. While the asphalt may be used alone, such as where it applied as a relatively thin film on existing paving structure, it is usually used in an asphalt concrete in which the asphaltic base material is mixed with a aggregate in an amount substantially in excess of the amount of the asphalt. Typically, an asphalt concrete may contain about 5-20 wt. % asphalt binder with the remainder being the aggregate material. The asphalt binder material may be modified through the use of polymers to produce polymer-modified asphalts and may further incorporate additional additives such as ground rubber, also called crumb rubber. It may also incorporate elastomeric-type polymers, such as polybutadiene, polyisoprene or polyisobutene rubber, polymethacrylate and ethylene propylene diene terpolymer. As described in the aforementioned SUPERPAVE booklet under the subheading “Aging Behavior,” asphalt can degenerate because of oxidation of its component compounds and devolatilization, in which volatile components gradually evolve from the asphalt. Thus, the asphalt paving materials, when first laid down, tend to be in a relatively resilient state in which they are impacted, but not fractured, under the stress imposed by vehicular traffic. With the passage of time and the release of the more volatile components from the asphalt, as well as oxidative hardening, the asphalt tends to age and become brittle. Thus, particularly in the case of heavily traveled paving surfaces, the asphalt concrete becomes less resilient and as the surfaces lose their resiliency, the pavement fractures under applied stress and the asphalt concrete becomes heavily fractured. The fractures occur initially at the surface where the asphalt tends to be most heavily devolatized and oxidized. Thus the pavement near the surface can be characterized as “dead” asphalt, while the lower portion of the asphalt has a higher volatiles content and thus retains some resiliency.
Once the asphalt pavement becomes “dead” where it largely loses its effectiveness as a pavement surface, the pavement can be broken up and removed. Alternatively, the asphalt concrete material can be left in place and treated by the addition of additional material, such as asphalt cement or hydraulic cement, and then recompressed to form a new paving structure. Typically, the new structure would be used as a base and covered with a relatively thin film of asphalt or possibly an asphalt concrete having relatively fine aggregate components.